Allophycocyanin (APC), highly fluorescent phycobiliprotein, is made up of alpha and beta subunits and is present as a trimer (αβ)3, which is unstable and susceptible to dissociation at low concentrations. The monomer, αβ, has a lower fluorescence quantum yield compared to the trimer and the maximal absorption is also shifted to 620 nm. The chemically cross-linked APC trimer is much more stable than the native APC trimer, but still retains the same spectroscopic properties as the native APC trimer. APC labeled streptavidin, primary and secondary antibodies have been widely used in applications such as flow cytometry, live cell staining, and immunofluorescent staining.